Probabilistic seismic hazard assessment of Bishkek, Kyrgyzstan, considering empirically estimated site effects

Abstract

<p>It is well known that variability in the surface geology potentially leads to the modification of earthquake-induced ground motion over short distances. Although this effect is of major importance when seismic hazard is assessed at the urban level, it is very often not appropriately accounted for. In this paper, we present a first attempt at taking into account the influence of the shallow geological structure on the seismic hazard assessment for Bishkek, Kyrgyzstan, using a proxy (Vs30) that has been estimated from in situ seismic noise array analyses, and considering response spectral ratios calculated by analyzing a series of earthquake recordings of a temporary seismic network. To highlight the spatial variability of the observed ground motion, the obtained results are compared with those estimated assuming a homogeneous Vs30 value over the whole urban area. The seismic hazard is evaluated in terms of peak ground acceleration (PGA) and spectral acceleration (SA) at different periods (frequencies). The presented results consider the values obtained for a 10% probability of exceedance in 50 years. The largest SA estimated considering a rock site classification of the area (0.43 g) is observed for a period of 0.1 s (10 Hz), while the maximum PGA reaches 0.21 g. When site effects are included through the Vs30 proxy in the seismic hazard calculation, the largest SA, 0.67 g, is obtained for a period of 0.3 s (about 3.3 Hz). In terms of PGA, in this case the largest estimated value reaches 0.31 g in the northern part of the town. When the variability of ground motion is accounted for through response spectrum ratios, the largest SA reaches a value as high as 1.39 g at a period of 0.5 s. In general, considering site effects in the seismic hazard assessment of Bishkek leads to an increase of seismic hazard in the north of the city, which is thus identified as the most hazardous part within the study area and which is more far away from the faults.</p>

Probabilistic seismic hazard assessment for Central Asia

“…On the other hand, the results presented for the most important cities, i.e., the results for the local scale, have to be seen as part of a regional large-scale study. For site-specific investigations of engineering interest, local conditions, especially concerning the local geological conditions, the regionalization, and more detailed attenuation relationships, need to be considered (see the paper of Ullah et al [2015], this issue).…”

An overview on the seismic microzonation and site effect studies in Central Asia

“…New approaches like cluster analysis can allow an even faster and more efficient microzonation with a higher spatial resolution by using seismological parameters alone. Hence, when the soil properties in terms of the average S-wave velocity of the uppermost 30 m (v s 30 ), assessed from seismic noise array analyses, and response spectral ratios from earthquake recordings, are adequately known, explicit site-specific consideration of the soil response in probabilistic seismic hazard assessment can provide soil-surface-hazard estimates which are more precise than those from simple attenuation equations for generic soil conditions (see Ullah et al [2015], this issue). The results highlight the under-estimation of seismic hazard relying only on rock site conditions or v s 30 .…”